For examination of the characteristics of lentivirus-induced pulmonary disease in an animal model, neonatal lambs were given intratracheal injections of high- and low-passage ovine lentivirus (OvLV) isolates. In 6 of 6 lambs inoculated with low-passage OvLV or OvLV from lung lavage fluid, lesions of lymphoid interstitial pneumonia (LIP) developed. In none of 7 lambs inoculated with a high-passage OvLV or 4 control lambs inoculated with medium alone or ultrafiltered lung fluid did lung lesions develop. Systemic distribution of lentivirus was greater and development of lentivirus antibody was more rapid in lambs inoculated with low-passage OvLV, compared with lambs inoculated with high-passage OvLV. The number of lymphocytes in bronchoalveolar lavage samples was increased in lambs with lymphoid interstitial pneumonia. The development of lymphoid interstitial pneumonia was markedly accelerated, in comparison with previous reports of experimentally induced lentivirus pneumonia in sheep. In lentivirus-inoculated lambs pulmonary lesions developed comparable to lymphoid interstitial pneumonia associated with the acquired immunodeficiency syndrome and other human benign lymphoid disorders of the lung. Similarities between the disease manifestations and virologic properties of OvLV and human T-cell lymphotropic virus III argue for the relevance of OvLV-induced disease as a model for human retrovirus diseases. The ability of OvLV to cause accelerated pulmonary disease in neonates may be due to age-related susceptibility factors that enhance the pathogenicity of lentiviruses.